In the dark of night in autumn, a hunter waits patiently for the skittering of a rotting rat to pervade its own shriveled ears. Small and nimble, yet uncanny in its walk, dragging its body along haphazardly if it has gone for more than a week without a meal, the mortifelis haunts the Stream when the moon has waned. Unnerving to most, even those fond of cats, these beasts are harmless to humans but unpleasant to be around, for their appearance and their smell, but also their constant, unending hunger. Their flesh seems to fall off as they go about their lives, leaving traces of tissue and rot behind as a calling card for the other creatures to identify them by. Their own rank scent is only a minor deterrent in the hunt, for the mortifelis can always find rotting meat, scavenging carrion to build up strength for its favored prey, the decessi. In active pursuit, they rely on speed and strength, with no worries about biting too hard or pushing themselves to harm; they are held together by magic, and have few physical limitations.

All decessi fear one creature above all else, the dreaded mortifelis that devours them by the thousands come the autumn harvest. The mortifelis were produced centuries ago at the Keep, experiments to imbue a living cat with the ability to kill the hordes of undead rats that flooded the countryside each year. The experiments were an astounding success initially, but soon the magi who had created them started noticing that the cats were behaving oddly. They refused to eat regular food and appeared to stop breathing - yet they were still alive. Their skin would slough off in putrescent chunks, even down to the bone. Their eyes would decay and their noses would shrivel up, their ears eaten away by maggots and other insects that feast on rotting flesh, yet the cats seemed unaffected, clinging to life in much the same way as the decessi. Given free reign like other creatures of the Keep, these cats would occasionally come to scratch or bite other cats, spreading their condition, and their love of dead flesh. In due time, the existing cats were relocated to far corners of the world where they wouldn't pose a threat to living cats, but occasionally they find themselves back at the Keep with the onslaught of decessi.